This invention relates generally to radio frequency control systems and more particularly to automatic frequency control (AFC) systems for single sideband receivers and still more particularly to split loop AFC systems to be used at frequencies designated for land mobile services.
It is well known by those skilled in the art that the tolerable frequency error, without significant loss of voice recognition or intelligibility, is approximately .+-.20 Hz in a single sideband radio system. Traditional means of controlling radio frequencies such as crystal frequency standards cannot alone maintain this close tolerance at frequencies greater than approximately 20 MHz. Typical crystal oscillator performance over the temperature extremes encountered in a mobile radio environment range from .+-.5 parts per million (ppm) to .+-.1 ppm. The cost of the .+-.1 ppm oscillator is inherently much greater than that of a .+-.5 ppm oscillator. Considering a UHF frequency of 860 MHz, these tolerances yield frequency errors of .+-.4.3 kHz and .+-.860 Hz respectfully.
To resolve this frequency tolerance problem a pilot signal, which is generally employed by the receiver to eliminate the frequency errors introduced by the crystal oscillators, may be transmitted with the single sideband signal. Typically, the receiver AFC employs a phase locked loop (PLL) which locks to the pilot signal. Various continuous PLL techniques have been widely adopted among single sideband receiver designers. Generally, PLL receiver AFC circuits are realized in a loop that includes a mixer, an intermediate frequency (IF) filter and gain, phase detector with associated reference oscillator, a loop filter, and a voltage controlled oscillator (VCO) which accepts a frequency control voltage from the loop filter and produces a local oscillator (LO) signal for the mixer.
When the VCO controlled by the phase lock loop is realized in a first (pre-IF) mixer of an SSB receiver the technique is generally known, among those skilled in the art, as a long loop AFC system. While long loop AFC systems are a contribution to the art they do, however, suffer phase lock dropouts due predominately to noise on the VCO control line often caused by fading of the received signal. A further deficiency of long loop AFC systems is the excessive pull-in times required to accomplish phase lock. This delay, primarily induced through the IF strip filtering, may produce undesirable effects and partial loss of the recovered signal.
Many single sideband receiver designers have utilized a technique of positioning the VCO controlled by the phase detector after the IF strip to eliminate the excessive pull-in times of a long loop AFC system. While this so called short loop provides very rapid pull-in times to accomplish phase lock it is not without drawbacks. Paramount among these drawbacks is the effect of sideband cutting wherein the intelligible information transmitted is partially lost in the IF filter if the received signal is not exactly centered in the IF passband.
Various permutations of long and short loop AFC systems have become known in the art and include dual or multiple bandwidth AFC systems. To eliminate noise on the VCO control line which may cause PLL dropouts, loop filters with selectible transfer functions were incorporated to narrow the bandwidth of the loop as the VCO closed in on the desired frequency. However, the multiple bandwidth phase lock loop systems may have further shortcomings such as poor demodulator performance when the received signal is subject to multipath signal propagation or inferior acquisition performance.
Attempting to resolve these conflicting deficiencies of the above described PLL AFC systems, McGeehan, J. P., and Sladen, J. P. introduced in a paper entitled "Elimination of False-Locking in Long Loop Phase-Lock Receivers" IEEE Trans. on Comm., pages 2391-2397 October, 1982, a split loop AFC system. The split loop AFC system, a hybrid of the long and short loop AFC systems, attempts to incorporate into one system the advantages described above for each of the independent systems. In VCO's, a first prior to the IF strip and a second following the IF strip. The first VCO provides the centering to eliminate sideband cutting that may be experienced in the short loop AFC systems, and the second PLL obtains rapid lock time by avoiding the delay through the IF filters. While the split loop AFC system is a significant advance in the art and does in fact incorporate certain advantages of the prior long and short loop AFC systems, there remain shortcomings common to all continuous phase locked loop AFC systems in general. One such problem is described by McGeehan, J. P. in a paper entitled "Problem of Speech Pulling and its Implementation for the Design of Phase-Locked SSB Radio Systems" IEE Proceedings Volume 128, No. 6 pages 361-369 November, 1981. In this article McGeehan describes phase locked loop false locking, and VCO variations due to speech information getting into the phase locked loop and attempting to pull the VCO off the desired frequency. In various implementations of SSB receivers the effects of speech pulling manifest themselves differently. For example, a tone-in-band SSB system may experience a form of intermodulation distortion, and pilot-carrier systems may see the effect of speech pulling emerging as a form of harmonic distortion.
The present invention discloses a method of solving the split loop shortcomings and enabling single sideband receivers to approach the performance of well established frequency modulation receivers.